Separation/concentration of liquid mixtures by a membrane separation method is an energy-saving method as compared with a separation technique such as distillation because it is not accompanied with phase change. Also, it does not cause the phase changes in the state of substances. Therefore, it has been widely utilized in many fields including a food field such as concentration of fruit juice and separation of beer enzyme; and recovery of organic substances from industrial wastewater. Treatment of water by a semipermeable membrane has become firmly established as an indispensable process supporting the most leading-edge technology.
For example, recovery of freshwater from seawater with semipermeable membrane utilizing the concentration difference between seawater and aqueous solution of high concentration has been expected as an energy-saving desalination process for seawater since consumption of energy thereby is small as compared with a conventional method wherein higher pressure than osmotic pressure is applied to seawater followed by permeating with a reverse osmosis membrane so as to recover freshwater from seawater. Particularly due to the fact that a hollow fiber type semipermeable membrane can make the membrane area per membrane module volume large in spite of its small flow rate of permeated water per unit membrane area as compared with a spiral wound type semipermeable membrane, it has advantages that the permeated water flow rate can be made large as a whole and that volume efficiency is very high whereby it has excellent compactness. Further, in such a case of water treatment wherein aqueous solution of high concentration is fed into a module and made to contact with an aqueous solution of low concentration via a hollow fiber type semipermeable membrane and concentration difference generated thereby is used as a driving force, there are advantages that concentration polarization on the membrane surface can be made small as compared with reverse osmosis and that lowering of the concentration difference can be suppressed.
Generally, the conventional hollow fiber type semipermeable membrane for reverse osmosis treatment is manufactured in such a method that a dope containing cellulose acetate is prepared as a polymer material, extruded from a nozzle into air, coagulated in aqueous solution, washed with water and shrunk by annealing by hot water. For example, Examples in the Patent Document 1 disclose a hollow fiber type semipermeable membrane prepared in such a manner that a dope containing cellulose triacetate as a polymer material is extruded, coagulated, washed with water and annealed by hot water of 85° C. for 20 minutes under no tension. According to the data of the Examples, it is shown that, when 0.2% aqueous solution of sodium chloride was used as a feed water and measurement was conducted under the pressure of 30 kg/cm2, the permeation flow rate and NaCl rejection rate of hollow fiber type semipermeable membrane were 230 L/m2/day and 99.85% (Example 1); 245 L/m2/day and 99.87% (Example 3); or 250 L/m2/day and 99.84% (Example 4), respectively. However, in the conventional hollow fiber type semipermeable membrane as disclosed in the Patent Document 1, a high salt rejection property can achieved because the membrane shrinkage is made big by means of an annealing treatment by hot water at high temperature. When it is used at low pressure, however, there is a problem that the water permeation property greatly lowers. Accordingly, its treating ability cannot be made high when, for example, high pressure is not applied including such a case wherein the semipermeable membrane is used for water treatment utilizing the concentration difference as a driving force.
Examples of an art for keeping both water permeability and separating property in high level in a hollow fiber type semipermeable membrane include Patent Documents 2 and 3. The Patent Document 2 discloses an art concerning a hollow fiber type semipermeable membrane module which is utilized for the separation of solid or solute from a liquid mixture. However, according to the properties of the hollow fiber membrane using cellulose triacetate shown in Table 1 of the Patent Document 2, the permeation flow rate (FR1) measured under operation pressure of 55 kg/cm2 is 22.6 to 91.5 L/m2·day and no high water permeability can be achieved in forward osmosis treatment.
Patent Document 3 discloses an art concerning a flat sheet type composite semipermeable membrane which is equipped, on a microporous support, with an active layer (thin membrane, skin layer) containing interfacial polymerized polyamide as a main component and which has both high salt rejection rate and high permeability. According to the description of Example 1, the semipermeable membrane disclosed in the Patent Document 3 has permeation flow rate of 1.0 m3/m2·day (1000 L/m2·day) when measured under an operation pressure of 7.5 kg/m2. However, this semipermeable membrane is in a form of flat membrane whereby, in a membrane module used for the actual water treatment wherein the concentration difference via semipermeable membrane is used as a driving force, it is difficult that aqueous solution of high concentration and aqueous solution of low concentration (freshwater) fed to the module are effectively and uniformly partitioned to the front surface of the membrane. In addition, in the area to which small flow rate of the aqueous solution is fed, concentration polarization on the membrane surface becomes particularly high. Accordingly, it is difficult to effectively ensure the concentration difference via the membrane whereby there is a disadvantage that efficiency of the water treatment cannot be made high. Moreover, in the membrane comprising such a polyamide material, there are disadvantages that it is inferior in its resistance to chlorine and that usable bactericidal chemicals are limited.
On the other hand, there has been a brisk demand from the consumers paying their importance to economy and compactness of a water treatment plant using a membrane method for improving the treating ability per membrane area even in the case of a hollow fiber type semipermeable membrane. In the case of water treatment wherein the concentration difference is used as a driving force, no high water permeability is achieved even if the conventional semipermeable membrane for lower pressure is used. As a result, it is the present situation that water production cost and installation space cannot be suppressed.
As mentioned hereinabove, it is the present situation that there is no cellulose acetate-based hollow fiber type semipermeable membrane which can achieve both water permeability and selectivity in high level and which can perform efficient water treatment utilizing the concentration difference between two liquids via a membrane, in a small installation space.